Cooling channels of this type are intended for cooling the free end of the blade since they enable a jet of cooling air to be delivered from the internal cooling passage towards the end of the blade at the top end of the outside face of the pressure side wall. This jet of air serves to “pump heat”, i.e. to reduce the temperature of the metal by absorbing heat from the core of the metal wall, and it also creates a film of cooling air that protects the ends of the blades on the pressure side.
Because of the high working speeds at the ends of such blades, and because of the temperatures to which the blades are subjected in operation, it is necessary to cool them so that their temperature remains lower than the temperature of the gas in which they work.
That is why it is conventional for blades to be hollow so as to enable them to be cooled by the air present in an internal cooling passage.
It is also known to provide the end of the blade with an open cavity that is also referred to as a “bathtub”: this shape for the end of a blade limits the areas facing each other between the end of the blade and the corresponding annular surface of the turbine casing, so as to protect the body of the blade against the damage caused by the blade possibly coming into contact with the annular segment.
Patent documents U.S. Pat. No. 6,231,307, EP 0 816 636, and FR 2 858 650 present such a hollow blade that it is also provided with cooling passages connecting the internal cooling passage with the outside face of the rim of the cavity beside the pressure side wall, these cooling channels opening out at their outlets in the outside face of the pressure side wall towards the tip of said rim.
Those cooling channels situated beside the pressure side wall thus enable a jet of air to exit from the internal cooling passage that is cooler than the air surrounding the pressure side wall with said jet of air forming a film of cooling air that is localized over the outside face of the pressure side wall, and that is sucked towards the suction side wall, passing over the end of the blade.
In patent document U.S. Pat. No. 6,231,307, those inclined cooling channels connect the internal cooling passage with the outside face of the rim of the cavity at the pressure side wall by being disposed (see FIG. 2 of that document) in such a manner as to pass through the end wall of the cavity and through the rim of the cavity level with the pressure side wall, passing via said cavity.
That solution thus requires a large thickness of material, whether for the end wall of the cavity or for the cavity rim, in order to avoid degrading the high temperature strength performance at the tip of the blade. In addition, that solution puts a very severe limit on the flow of cooling air that reaches the tip of the rim, since the major fraction of the flow leaves the internal cooling passage via the first segments of the cooling channels and penetrates directly into the cavity without reaching the outside face of the pressure side wall.
The solution of document EP 0 816 636, as can be seen in FIG. 5 of that document, consists in placing those cooling channels in such a manner that they pass through the pressure side wall opening out into the outside face of said pressure side wall level with the base of a cavity rim.
That solution likewise requires a large thickness of material, whether for the end wall of the cavity or for the cavity rim, in order to avoid degrading the high temperature strength performance at the tip of the blade.
Document FR 2 858 650 proposes a solution (see FIG. 5) that consists in providing reinforcement of material between the rim and the end wall of the cavity along at least a fraction of the pressure side wall, whereby said rim is enlarged at its base adjacent to said end wall so that the cooling channels open out close to the tip of the rim without degrading the high temperature strength of the blade. In that way, by having reinforcing material, the cooling channels can thus open out closer to the tip of the rim without changing the distance between said cooling channels and the end wall of the cavity.
However, given the ever-increasing operating temperatures of turbines, those solutions no longer make it possible to provide a hollow blade with the end of the blade being cooled in satisfactory manner.
In order to maintain sufficient high temperature strength around the cooling channels, making use of large wall thicknesses leads to the moving wheel(s) of the turbine being made much heavier. Consequently, since the thicknesses of material are large, the more the temperature rises because cooling is not so fast, the more these large thicknesses of material prevent sufficient cooling at the tip of the blade to enable the turbine to operate at the desired higher temperatures.
It should be observed that if cooling is insufficient at the end of the blade, local burning can take place that can lead to metal being lost, thereby increasing clearances, and thus harming the aerodynamic efficiency of the turbine. Likewise, when the rim of the cavity sees its temperature increase excessively, there is observed to be a risk of burning with damage to the metal wall.